1. Field of the Invention
The invention relates to a variable wavelength laser light source, and particularly, to a variable wavelength laser light source capable of causing laser oscillation by use of an external resonator composed of a high reflection film face provided on the back end of a laser diode, and an optical return path for enabling outgoing light from the laser diode to return thereto.
2. Prior Art
It is well known that laser oscillation is caused to occur by means of an external resonator formed between one end face of a Fabry-Perot laser diode, provided with a non-reflection film, and the other end face thereof by causing a light beam to return to the laser diode by use of a device having wavelength selectivity provided that gain condition and phase condition overcome losses such as reflection loss, scattering loss, and the like.
Thus, a variable wavelength laser light source is set up by causing a laser beam of optional wavelength selected from outgoing light sent out from a laser diode capable of causing laser oscillation to return to the laser diode.
FIGS. 3 and 4 are schematic illustration showing examples of conventional variable wavelength laser light sources, respectively. Firstly, referring to FIG. 3, a laser diode 8a is provided with a low reflection film 8b on one end face, and a non-reflection film 8c on the other end face. Laser light outgoing from the laser diode 8a is converted to a collimated light beam by a lens 9b, and in front of the lens 9b, a total reflection mirror 13 is disposed via a light-variable band-pass filter 12. The total reflection mirror 13 and the low refection film 8b constitute an external resonator.
In front of the low refection film 8b, a lens 9a is disposed such that a laser beam transmitting therethrough propagates in an optical fiber 10, and is sent out through an output port 11.
Now, referring to FIG. 4, another example of a conventional variable wavelength laser light source has the same construction as that shown in FIG. 3 except that the light-variable band-pass filter 12 and the total reflection mirror 13 shown in FIG. 3 are dispensed with, and instead, a grating 14 is provided. Parts common to FIGS. 3 and 4 are denoted by the same reference numerals to avoid duplication in explanation of the construction.
Next, referring to FIG. 3, operation of the conventional variable wavelength laser light source is described hereinafter. The outgoing light from the laser diode 8a is converted to the collimated light beam by the lens 9b, and reflected from the total reflection mirror 13 after going through the light-variable band-pass filter 12, transmitting light of specific wavelength. The collimated light beam propagates after changing its direction by 180.degree. when reflected from the total reflection mirror 13, and falls on the laser diode 8a after passing through the light-variable band-pass filter 12 again. The light beam after entering the laser diode 8a is bounced back by the surface of the low refection film 8b having several % of reflectivity, and returns to the laser diode 8a again. The low refection film 8b and the total reflection mirror 13 constitute the external resonator, causing laser oscillation to occur.
The light beam with laser oscillation occurring travels via the lens 9a through the optical fiber 10, and outputted to the output port 11. This construction functions as the variable wavelength laser light source by varying wavelength transmitting the light-variable band-pass filter 12.
In the case of another conventional variable wavelength laser light source shown in FIG. 4, the construction thereof is similar to that shown in FIG. 3. The outgoing light from the laser diode 8a converted to the collimated light beam by the lens 9b falls on the grating 14, and after selection of wavelength by the grating utilizing diffraction of light waves, enters again into the laser diode 8a via the lens 9b. Thereafter, as shown in FIG. 3, the light beam with laser oscillation travels through the optical fiber 10 after transmitting through the lens 9a, and outputted to the output port 11.
With the conventional variable wavelength laser light sources as shown in FIGS. 3 and 4, a high ratio of light returning to an active layer of the laser diode is required to cause laser oscillation to occur. In addition, a light emitting region of the active layer of the laser diode is minuscule and on the order of 1 .mu.m. Accordingly, optical components thereof need to be disposed with high precision in relation to each other.